Flat Engined Marvels

First patented in 1896 by the father of the modern internal combustion engine, Karl Benz, the concept of a “flat” engine has been around since the concept of a powered four-wheeled automobile. Over the course of automotive history, there have been numerous instances where a flat engine has been used, either for performance, handling, or space saving reasons. While it's not exactly the mainstream engine of choice for manufacturers, those who have stuck to it, have perfected the design over the years, with manufacturers like Subaru and Porsche claiming the flat engine to be their respective specialties.   
There is a slight distinction between flat engines and “Boxer” engines, and the difference lies in how the pistons move.
If the pistons in a flat engine move alternate to each other and are connected to the crankshaft by shared crankpins, it is a regular flat engine, as used in the early, air-cooled Volkswagen Beetles, as well as the Porsche 911. Interesting fact, the Chevrolet Corvair (one of the most misunderstood American cars in history, despite being brilliantly engineered) used a flat engine placed at the back of the car, which was unheard of in the muscle car heyday in America. Unsurprisingly, Chevrolet went to rear placed flat engine maestros Porsche to engineer the car.

Boxer engines have pistons which move outward and inward simultaneously, and have individual crank pins connecting each piston to the crankshaft. The name “boxer” is derived from this simultaneous motion, because the pistons look like a couple of boxers punching their gloves together before a fight. Subaru loves the boxer engine so much, it's a permanent feature on every single model in their lineup, and its this uniqueness which sets Subaru owners apart from the rest of the crowd.
Why is the flat engine cool? Well, there's the immensely low center of gravity, because flat engines are hunkered down and less top heavy than other engine layouts, which makes a flat-engined car a force to be reckoned with in the corners. The Boxer engine saves weight because the it is inherently balanced by the laws of physics, so the internals don't require counterweights to keep opposing moments (turning forces) in check. The infamous Boxer burble is enough to wet the pants of hormonal teenagers and turn them into lifelong Subaru fanatics, and beyond all of that, the strength and durability of the engine makes it a favourite of aftermarket tuners. A Subaru Boxer engine is rumoured to last a lifetime if you treat it right.
Why don't more manufacturers take on the flat engine if it offers so many benefits? Well, these engines tend to be thirsty, and there are massive costs involved in developing a Boxer from scratch. This is probably why Toyota partnered up with Subaru when they decided to equip their GT86/Scion FRS/Subaru BRZ with a boxer, to make use of the superior performance of the Boxer.
Here's to a lovely engine and properly engineered vehicles. Much respect.